1. Field
This invention relates generally to millimeter wave imaging systems and, more particularly, to a millimeter wave imaging system including a highly-integrated millimeter wave focal plane radiometer array.
2. Description of Related Art
Generation of images responsive to detected millimeter waves (radiation having wavelengths in approximately the 1 cm-1 mm range, that is, frequencies between 30 GHz and 300 GHz) reflected from or emitted by objects in a field of view is desired in many applications. This is largely because millimeter waves penetrate many materials that are opaque to visible and infrared radiation, enabling high-resolution imaging of scenes that were previously invisible. For example, millimeter wave imagers could provide landing assistance to aircraft for runways obscured by fog. Additionally, millimeter wave imagers could provide images of weapons concealed beneath clothing, since human bodies and metal objects have different optical properties at millimeter wavelengths.
Since all objects reflect and emit millimeter waves, passive imaging can be used to detect the natural millimeter wave emissions or reflections from objects or people. The emissivity range of objects at millimeter waves is very large, approximately ten times greater than the range provided by infrared, so high contrast images can be made using existing blackbody radiation. A passive imager uses sensitive receivers to distinguish small differences in millimeter wave emissions. The emitted radiation is processed by the detector which converts the millimeter wave emissions down to a video signal. The strength of the video signal is roughly proportional to the power level in the emitted radiation.
Creating an image from emitted millimeter wave radiation has been historically difficult due to the lack of small, sensitive millimeter wave detectors that can be easily arrayed. Early versions of millimeter wave imaging systems used mechanical or electronic scanning of the millimeter wave sensor. Mechanical systems physically move a sensor through a range of azimuths, elevations, or both, defining a field of view. Such systems are complex and subject to failure. Electronic scanning typically requires employment of electronic phase shifting or switching techniques which are relatively complex to implement at millimeter wave frequencies.
Later generations of millimeter wave imaging systems used focal plane arrays of millimeter wave detectors. These systems are characterized by the use of conventional two dimensional integration of electronics using circuit board techniques. U.S. Pat. No. 4,910,523 issued Mar. 20, 1990 to R. G. Huguenin, et al, discloses a focal plane array comprising multiple circuit boards disposed in a horizontal direction where each circuit board has multiple detectors disposed in a vertical direction. U.S. Pat. No. 5,438,336 issued Aug. 1, 1995 to P. S. C. Lee, et al, also discloses a focal plane array for millimeter wave imaging. In Lee, an array of pixels are used for detection of millimeter wave images. Each pixel comprises an antenna, a low noise amplifier, a band pass filter and a video detector. However, Lee discloses using discrete parts for these elements, so the level of integration occurs at the level of a single pixel, and the pixels would be constructed using circuit board techniques. Lee also discloses that signal processing of the detected signal is done externally to the individual pixels.
A millimeter wave video camera operating at 89 GHz is disclosed by R. T. Kurado et al., in xe2x80x9cLarge Scale W-Band Focal Plane Array Developments for Passive Millimeter Wave Imaging,xe2x80x9d SPIE Conference on Passive Millimeter-Wave Imaging Technology, April 1998, pp. 57-62. The millimeter wave imager disclosed by Kurado is approximately 30 inches on a side and is assembled using hybrid circuit-card techniques. The focal plane detector within the disclosed imager uses 1040 MMICs and antennas, along with 15,860 resistors, capacitors, and silicon integrated circuits interconnected by 36,920 wire bonds.
U.S. Pat. No. 5,237,334 issued Aug. 17, 1993 to W. M. Waters discloses a focal plane antenna array comprising a plurality of conical horns and circular waveguides. The waveguide array is constructed by perforating an aluminum plate to provide passages defining the waveguides. Diodes are then manually assembled into each aperture to provide detection of millimeter waves. Signal processing of the detected millimeter waves is done by a separate signal processing module. Conventional wiring leads are used to connect the array of detectors to the signal processing module.
All of the systems discussed above use conventional circuit board integration where components are serially picked-and-placed into a primarily two dimensional structure. These systems suffer from a large size and time-consuming serial assembly resulting from the use of conventional two dimensional module-style integration to implement a dense, inherently three-dimensional array. Such systems also exhibit degraded system performance due to unnecessarily long interconnections between electronic components. Construction of large, costly detector elements may force the use of sparsely-populated arrays with image resolution that falls well short of theoretical limits. Attempts to save space by sharing and multiplexing one video processor among a number of detector elements, or by using mechanically scanned optics, results in slow refresh rates that are inadequate for real-time video. These limitations have prevented widespread deployment of millimeter wave imaging technology.
In light of the discussion above, there exists a need in the art for a compact focal plane array for millimeter wave imaging. Specifically, such an array should maximize integration of the elements required for millimeter wave imaging while minimizing reliance on conventional circuit board assembly techniques. Such an array should also allow for minimum spacing of array elements to achieve maximum resolution as well as minimizing circuit interconnection lengths.
It is an object of the present invention to provide a compact focal plane array for millimeter wave imaging that can be assembled with a minimal reliance on conventional circuit board assembly techniques. It is a further object of the present invention to allow elements of a millimeter wave focal plane array to be spaced to achieve maximum resolution of an image processed by the array. It is another object of the present invention to provide the capability for real-time video based on detected millimeter wave radiation.
The present invention comprises a three-dimensional, integrated focal plane radiometer array structure for millimeter wave imaging. The structure comprises one or more layers of polymer films containing encapsulated semiconductor devices, transmission lines, and circuit interconnects which are positioned on top of a substrate. The substrate contains multiple integrated circuits interconnected and connected to the semiconductor devices in the layers above. The top layer of the structure contain antenna elements which have been disposed upon a dielectric. The antenna elements are connected to the semiconductor devices in the layers below. The small size of the semiconductor devices and the integrated circuits allows the antenna elements to be closely spaced and minimizes the interconnection lengths between the antenna elements, the semiconductor devices, and the integrated circuits. The small spacing of antenna elements allows the focal plane array to produce a very high resolution image. Minimization of interconnection lengths provides that system noise is reduced and allows a real-time update rate to be used in imaging a scene.
A first embodiment of the present invention is provided by a substrate comprising a plurality of integrated circuits; one or more receptacle layers positioned on top of the substrate, the receptacle layers each comprising a plurality of microwave monolithic integrated circuits encapsulated in a polymer film, the plurality of the microwave monolithic integrated circuits connecting to the plurality of the integrated circuits by vertical interconnects; and an antenna carrying layer comprising: a dielectric layer and a plurality of antenna elements positioned on top of the dielectric layer, the plurality of antenna elements connecting to the plurality of the microwave monolithic circuits by vertical interconnects through the dielectric.
A second embodiment of the present invention is provided by a focal plane array structure comprising: a semiconductor wafer; a plurality of integrated circuits formed within the semiconductor wafer; one or more receptacle layers positioned on top of the silicon wafer, each receptacle layer comprising a plurality of microwave monolithic integrated circuits encapsulated in a polymer film; and an antenna carrying layer positioned on top of the receptacle layer, the antenna carrying layer comprising: a dielectric layer and a plurality of antenna elements positioned on top of the dielectric layer.
The small size of the microwave imaging array provided by the present invention allows for the deployment of a complete millimeter wave imaging system within a small, preferably handheld, enclosure. Such a millimeter wave imaging system comprises: a millimeter wave focal plane array producing full image video, said array comprising: a substrate containing a plurality of integrated circuits; one or more receptacle layers, each receptacle layer comprising a plurality of integrated circuits encapsulated in a polymer film; and an antenna carrying layer; and a lens directing the viewed scene onto the focal plane array; a processing unit receiving the full image video and producing a display video signal; and a video display creating a visual display from the display video signal.